Light source device

ABSTRACT

A light source device having a light guide ( 3 ) for guiding light emitted from a primary light source and emitting light obliquely from a light emission face ( 33 ) thereof, and a light deflecting device ( 4 ) disposed so as to be adjacent to the light emission face. Plural elongated prisms are arranged in parallel to one another at a light incident surface ( 41 ) of the light deflecting device. Virtual elongated prisms (I) each having a vertical angle θ of 50 to 70° arranged in the same arrangement pitch as the elongated prisms are assumed so that the peak emission light in the light emission distribution of light emitted from the light emission face is incident on one virtual prism face of the virtual elongated prism, internally totally reflected by another virtual prism face and then emitted in a desired direction from a light emission surface ( 42 ) of the light deflecting device. A prism face of each elongated prism at the far side from the primary light source is designed to have a convex surface shape with respect to the shape of the virtual elongated prism. The value (r/P) of the radius of curvature r normalized by the arrangement pitch P is equal to 7 to 30, and the ratio (d/P) of the maximum distance between the convex-surface-shaped prism face of the elongated prism and the virtual prism face to the arrangement pitch P is equal to 0.05 to 5%.

FIELD OF THE INVENTION

The present invention relates to an edge-lighting type light sourcedevice constituting a liquid crystal display device or the like which isused as a display device for a portable note-type personal computer, aliquid crystal television or the like, and particularly to animprovement of a light deflecting device arranged at a light emissionface side of a light guide.

DESCRIPTION OF THE RELATED ART

Recently, color liquid crystal display devices have been broadly used asmonitors for a portable note-type personal computer, a personalcomputer, etc. or as display devices for a liquid crystal television, avideo-installed liquid crystal television, etc. in various fields.Further, increase of the amount of information to be processed,diversity of needs, requirements of supporting multimedia are positivelypromoting enlargement of the screens and enhancement of high definitionin the liquid crystal display devices.

A liquid crystal display device is basically constructed by a back lightportion and a liquid crystal device portion. As the back light portionis known an under-lighting type back light portion in which a lightsource is disposed just under the liquid crystal device portion, or anedge-lighting type back light portion in which a light source isdisposed so as to face the side end face of a light guide. Theedge-lighting type is more frequently used from the viewpoint of thecompact design of the liquid crystal display device.

A display device having a relatively small screen size and a relativelynarrow viewing range in the viewing direction (hereinafter referred toas “viewing angle”), for example, a liquid crystal display device usedas a display portion for a cellular phone or the like, has used anedge-lighting type back light portion that is designed from theviewpoint of reduction of power consumption such that the expandingangle of light flux emitted from the screen is reduced to be as less aspossible so that the light can be concentratively emitted to a requiredangle range to effectively use the amount of light emitted from aprimary light source.

For example, the applicant of this application has proposed a lightsource device which has a limited viewing angle and emits lightconcentratively to a relatively narrow range to enhance effective use ofthe light amount of the primary light source and reduce the powerconsumption in Japanese Patent Application No. 2000-265574 (not priorart). In the light source device of this application, there is used aprism sheet having prism surfaces formed on both the surfaces thereof soas to be disposed adjacent to the light emission face of a light guide.In this double-sided prism sheet, plural elongated prisms arranged inparallel to one another are formed on each of a light incident surface(one surface of the prism sheet) and a light emission surface (the othersurface of the prism sheet), the directions of the elongated prisms arecoincident between the light incident surface and the light emissionsurface, and the elongated prisms of the light incident and lightemission surfaces are disposed in corresponding positional relationshipwith each other. Accordingly, light that is emitted from the lightemission face of the light guide while distributed in a proper anglerange and has a light emission peak in a direction oblique to the lightemission face is incident from one prism face on the light incidentsurface of the prism sheet, internally reflected by the other prism faceand further made to suffer a refractive action of the prism on the lightemission surface of the prism sheet, thereby concentratively emittingthe light to a relatively narrow required direction.

With the light source device described above, the concentrative lightemission in the narrow angle range can be performed. However, in thisapparatus, it is required that plural elongated prisms arranged inparallel to one another are disposed on both the surfaces of the prismsheet used as a light deflecting device so that the prism direction iscoincident between the light incident surface and the light emissionsurface of the prism sheet and also the elongated prisms on the lightincident and emission surfaces are disposed in corresponding positionalrelationship with each other. Therefore, the molding process iscomplicated.

Furthermore, there is proposed a prism sheet provided with elongatedprisms each having a prism face of convex surface shape at a side farfrom the light source in JP(A)-9-507584, JP(A)-9-105804, JP(A)-11-38209and JP(A)-2000-35763. This prism sheet is intended to enhanceconcentrative and directive properties of the light when the prism sheetis used for deflecting the light emitted from the light guide. However,the prism faces of convex surface shape described in the above patentpublications are relatively too large or too small in the radius ofcurvature, and therefore it is not sufficient to narrow the distributionof light emitted from the light guide, or it excessively narrow thedistribution or often broaden the distribution. In addition, withrespect to the light guides described in the above patent publications,they have light emission function which achieve not so high directivityin the emitted light distribution and therefore the emitted lightbecomes to have relatively broad distribution, so that the sufficientenhancement in luminance cannot be achieved even if the light emittedfrom the light guide is concentrated with use of the prism sheet.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a lightsource device with which a distribution of emitted light is controlledto be very narrow, a using efficiency of the light amount of a primarylight source can be enhanced (that is, the efficiency at which lightemitted from the primary light source is concentratively emitted to arequired viewing direction is high), the luminance is very high andimage quality can be easily enhanced with a simple construction.

In order to attain the above object, according to the present invention,there is provided a light source device comprising: a primary lightsource; a light guide having a light incident face on which lightemitted from the primary light source is incident, guiding the incidentlight, and having a light emission face from which the incident light isemitted; and a light deflecting device disposed so as to be adjacent tothe light emission face of the light guide, wherein the light deflectingdevice has a light incident surface disposed so as to face the lightemission face of the light guide and a light emission surface at theopposite side to the light incident surface, plural elongated prismsarranged in parallel to one another are formed on the light incidentsurface of the light deflecting device, each of the elongated prisms hastwo prism faces, and when there are assumed plural virtual elongatedprisms arranged at the same arrangement pitch as the elongated prisms ofthe light deflecting device, each virtual elongated prism having avertical angle θ of 50 to 70′, and the peak emission light in the lightemission distribution of light emitted from the light emission face ofthe light guide being incident on one virtual prism face of the virtualelongated prism, internally totally reflected by another virtual prismface and then emitted in a desired direction from the light emissionsurface, at least prism face of each elongated prism at the far sidefrom the primary light source is designed to have a convex surface shapewith respect to the shape of each of the virtual elongated prisms, theconvex surface shape being set to such a condition that the value (r/P)of the radius of curvature r normalized by the arrangement pitch P ofthe elongated prisms is equal to 7 to 30.

In an aspect of the present invention, the ratio (d/P) of the maximumdistance between the convex-surface-shaped prism face of each of theelongated prisms of the light deflecting device and the virtual prismface of each virtual elongated prism to the arrangement pitch P of theelongated prisms is equal to 0.05 to 5%. In an aspect of the presentinvention, a slant angle of a virtual prism face of each of the virtualelongated prisms at the near side to the primary light source is 45° ormore. In an aspect of the present invention, each of the virtualelongated prisms has an equilateral triangular cross-section. In anaspect of the present invention, the light emission face of the lightguide and/or the back surface of the light guide at the opposite side tothe light emission face is a surface having a directive light emissionfunction. In an aspect of the present invention, an average slant angleof the surface having the directive light emission function is 0.5 to15°. In an aspect of the present invention, the surface having thedirective light emission function is a roughened surface or a surfacehaving many elongated lens arranged thereon.

In order to attain the above object, according to the present invention,there is also provided a light source device comprising: a primary lightsource; a light guide having a light incident face on which lightemitted from the primary light source is incident, guiding the incidentlight, and having a light emission face from which the incident light isemitted; and a light deflecting device disposed so as to be adjacent tothe light emission face of the light guide, wherein the light deflectingdevice has a light incident surface disposed so as to face the lightemission face of the light guide and a light emission surface at theopposite side to the light incident surface, plural elongated prismsarranged in parallel to one another are formed on the light incidentsurface of the light deflecting device, each of the elongated prisms hastwo prism faces, and when there are assumed plural virtual elongatedprisms arranged at the same arrangement pitch as the elongated prisms ofthe light deflecting device, each virtual elongated prism having avertical angle θ of 50 to 70°, and the peak emission light in the lightemission distribution of light emitted from the light emission face ofthe light guide being incident on one virtual prism face of the virtualelongated prism, internally totally reflected by another virtual prismface and then emitted in a desired direction from the light emissionsurface, at least prism face of each elongated prism at the far sidefrom the primary light source is designed to have a convex surface shapewith respect to the shape of each of the virtual elongated prisms, theratio (d/P) of the maximum distance between the convex-surface-shapedprism face and the virtual prism face of each virtual elongated prism tothe arrangement pitch P of the elongated prisms is equal to 0.2 to 2%.

In an aspect of the present invention, each of the elongated prisms hasa common base with each virtual elongated prism, and the convex surfaceshape is set to a substantially cylindrical surface shape under thecondition that the value (r/P) of the radius of curvature r normalizedby the arrangement pitch P of the elongated prisms is equal to 2 to 80.In an aspect of the present invention, a slant angle of a virtual prismface of each of the virtual elongated prisms at the near side to theprimary light source is 45° or more. In an aspect of the presentinvention, each of the virtual elongated prisms has an equilateraltriangular cross-section. In an aspect of the present invention, thelight emission face of the light guide and/or the back surface of thelight guide at the opposite side to the light emission face is a surfacehaving a directive light emission function. In an aspect of the presentinvention, an average slant angle of the surface having the directivelight emission function is 0.5 to 15°. In an aspect of the presentinvention, the surface having the directive light emission function is aroughened surface or a surface having many elongated lens arrangedthereon.

In order to attain the above object, according to the present invention,there is also provided a light source device comprising: a primary lightsource; a light guide having a light incident face on which lightemitted from the primary light source is incident, guiding the incidentlight, and having a light emission face from which the incident light isemitted; and a light deflecting device disposed so as to be adjacent tothe light emission face of the light guide, wherein the light deflectingdevice has a light incident surface disposed so as to face the lightemission face of the light guide and a light emission surface at theopposite side to the light incident surface, plural elongated prismsarranged in parallel to one another are formed on the light incidentsurface of the light deflecting device, each of the elongated prisms hastwo prism faces, and at least one prism face of each elongated prism ofthe light deflecting device is designed in such a convex surface shapeas to have a slant angle larger than the slant angle of at least a partof a virtual prism face of a virtual elongated prism to the lightemission surface in an area nearer to the light emission surface thanthe position at which virtual light skirting the apex portion of aneighboring virtual elongated prism to pass in the direction of the peakemission light of the light emission disibibution of light emitted fromthe light emission face of the light guide is internally totallyreflected by the virtual prism face of the virtual elongated prism.

In an aspect of the present invention, each of the elongated prisms hasa common base with each virtual elongated prism, and the convex surfaceshape is set to a substantially cylindrical surface shape under thecondition that the value (r/P) of the radius of curvature r normalizedby the arrangement pitch P of the elongated prisms is equal to 2 to 80.In an aspect of the present invention, the ratio (d/P) of the maximumdistance between the convex-surface-shaped prism face of each of theelongated prisms of the light deflecting device and the virtual prismface of each virtual elongated prism to the arrangement pitch P of theelongated prisms is equal to 0.05 to 5%. In an aspect of the presentinvention, a slant angle of a virtual prism face of each virtualelongated prism at the near side to the primary light source is 45° ormore. In an aspect of the present invention, each virtual elongatedprism has an equilateral triangular cross-section. In an aspect of thepresent invention, the light emission face of the light guide and/or theback surface of the light guide at the opposite side to the lightemission face is a surface having a directive light emission function.In an aspect of the present invention, an average slant angle of thesurface having the directive light emission function is 0.5 to 15°. Inan aspect of the present invention, the surface having the directivelight emission function is a roughened surface or a surface having manyelongated lens arranged thereon.

In order to attain the above object, according to the present invention,there is also provided a light source device comprising: a primary lightsource; a light guide having a light incident face on which lightemitted from the primary light source is incident, guiding the incidentlight, and having a light emission face from which the incident light isemitted; and a light deflecting device disposed so as to be adjacent tothe light emission face of the light guide, wherein the light deflectingdevice has a light incident surface disposed so as to face the lightemission face of the light guide and a light emission surface at theopposite side to the light incident surface, plural elongated prismsarranged in parallel to one another are formed on the light incidentsurface of the light deflecting device, each of the elongated prisms hastwo prism faces, and when there are assumed plural virtual elongatedprisms arranged at the same arrangement pitch as the elongated prisms ofthe light deflecting device, each virtual elongated prism having avertical angle θ of 50 to 70°, and the peak emission light in the lightemission distribution of light emitted from the light emission face ofthe light guide being incident on one virtual prism face of the virtualelongated prism, internally totally reflected by another virtual prismface and then emitted in a desired direction from the light emissionsurface, at least one prism face of each elongated prism is designed tohave a convex surface shape with respect to the shape of each of thevirtual elongated prisms, and, the light emission face of the lightguide and/or the back surface of the light guide at the opposite side tothe light emission face is a surface having a directive light emissionfunction and having an average slant angle of 0.5 to 15°.

In an aspect of the present invention, each of the elongated prisms hasa common base with each virtual elongated prism, and the convex surfaceshape is set to a substantially cylindrical surface shape under thecondition that the value (r/P) of the radius of curvature r normalizedby the arrangement pitch P of the elongated prisms is equal to 2 to 80.In an aspect of the present invention, the ratio (d/P) of the maximumdistance between the convex-surface-shaped prism face of each of theelongated prisms of the light deflecting device and the virtual prismface of each virtual elongated prism to the arrangement pitch P of theelongated prisms is equal to 0.05 to 5%. In an aspect of the presentinvention, a slant angle of a virtual prism face of each of the virtualelongated prisms at the near side to the primary light source is 45° ormore. In an aspect of the present invention, each of the virtualelongated prisms has an equilateral triangular cross-section. In anaspect of the present invention, the surface having the directive lightemission function is a roughened surface or a surface having manyelongated lens arranged thereon.

In order to attain the above object, according to the present invention,there is also provided a light source device comprising: a primary lightsource; a light guide having a light incident face on which lightemitted from the primary light source is incident, guiding the incidentlight, and having a light emission face from which the incident light isemitted; and a light deflecting device disposed so as to be adjacent tothe light emission face of the light guide, and provided with a lightincident surface having plural elongated prisms which are arranged inparallel to one another so as to confront the light emission face of thelight guide, and a light emission surface at the opposite side to thelight incident surface, wherein the half-width B of the light emissiondistribution of light emitted from the light emission face of the lightguide within a plane vertical to the light incident face and the lightemission face is equal to or less than 36°, and the half-width A of thelight emission distribution of light emitted from the light emissionsurface of the light deflecting device within a plane vertical to thelight incident face and the light emission face is equal to 30 to 95% ofthe half-width B.

In an aspect of the present invention, the half-width A of the lightemission distribution of light emitted from the light emission surfaceof the light deflecting device within a plane vertical to the lightincident face and the light emission face is equal to 30 to 80% of thehalf-width B. In an aspect of the present invention, the half-width A ofthe light emission distribution of light emitted from the light emissionsurface of the light deflecting device within a plane vertical to thelight incident face and the light emission face is equal to 30 to 70% ofthe half-width B. In an aspect of the present invention, the half-widthA of the light emission distribution of light emitted from the lightemission surface of the light deflecting device within a plane verticalto the light incident face and the light emission face is 5 to 25°.

In an aspect of the present invention, each of the elongated prisms hastwo prism faces, and when there are assumed plural virtual elongatedprisms arranged at the same arrangement pitch as the elongated prisms ofthe light deflecting device, each virtual elongated prism having avertical angle θ of 50 to 70°, and the peak emission light in the lightemission distribution of light emitted from the light emission face ofthe light guide being incident on one virtual prism face of the virtualelongated prism, internally totally reflected by another virtual prismface and then emitted in a desired direction from the light emissionsurface, at least one prism face of each elongated prism is designed tohave a convex surface shape with respect to the shape of each of thevirtual elongated prisms.

In an aspect of the present invention, each of the elongated prisms hasa common base with each virtual elongated prism, and the convex surfaceshape is set to a substantially cylindrical surface shape under thecondition that the value (r/P) of the radius of curvature r normalizedby the arrangement pitch P of the elongated prisms is equal to 2 to 80.In an aspect of the present invention, the ratio (d/P) of the maximumdistance between the convex-surface-shaped prism face of each of theelongated prisms of the light deflecting device and the virtual prismface of each virtual elongated prism to the arrangement pitch P of theelongated prisms is equal to 0.05 to 5%. In an aspect of the presentinvention, a slant angle of a virtual prism face of each of the virtualelongated prisms at the near side to the primary light source is 45° ormore. In an aspect of the present invention, each of the virtualelongated prisms has an equilateral triangular cross-section. In anaspect of the present invention, the light emission face of the lightguide and/or the back surface of the light guide at the opposite side tothe light emission face is a surface having a directive light emissionfunction. In an aspect of the present invention, an average slant angleof the surface having the directive light emission function is 0.5 to15°. In an aspect of the present invention, the surface having thedirective light emission function is a roughened surface or a surfacehaving many elongated lens arranged thereon.

In order to attain the above object, according to the present invention,there is also provided a light source device comprising: a primary lightsource; a light guide having a light incident face on which lightemitted from the primary light source is incident, guiding the incidentlight, and having a light emission face from which the incident light isemitted; and a light deflecting device disposed so as to be adjacent tothe light emission face of the light guide, and provided with a lightincident surface having plural elongated prisms which are arranged inparallel to one another so as to confront the light emission face of thelight guide, and a light emission surface at the opposite side to thelight incident surface, wherein the half-width B of the light emissiondistribution of light emitted from the light emission face of the lightguide within a plane vertical to the light incident face and the lightemission face is more than 36°, and the half-width A of the lightemission distribution of light emitted from the light emission surfaceof the light deflecting device within a plane vertical to the lightincident face and the light emission face is equal to 30 to 70% of thehalf-width B.

In an aspect of the present invention, the half-width A of the lightemission distribution of light emitted from the light emission surfaceof the light deflecting device within a plane vertical to the lightincident face and the light emission face is equal to 30 to 60% of thehalf-width B. In an aspect of the present invention, the half-width A ofthe light emission distribution of light emitted from the light emissionsurface of the light deflecting device within a plane vertical to thelight incident face and the light emission face is equal to 30 to 50% ofthe half-width B. In an aspect of the present invention, the half-widthA of the light emission distribution of light emitted from the lightemission surface of the light deflecting device within a plane verticalto the light incident face and the light emission face is less than 25°.

In an aspect of the present invention, each of the elongated prisms hastwo prism faces, and when there are assumed plural virtual elongatedprisms arranged at the same arrangement pitch as the elongated prisms ofthe light deflecting device, each virtual elongated prism having avertical angle θ of 50 to 70°, and the peak emission light in the lightemission distribution of light emitted from the light emission face ofthe light guide being incident on one virtual prism face of the virtualelongated prism, internally totally reflected by another virtual prismface and then emitted in a desired direction from the light emissionsurface, at least one prism face of each elongated prism is designed tohave a convex surface shape with respect to the shape of each of thevirtual elongated prisms.

In an aspect of the present invention, each of the elongated prisms hasa common base with each virtual elongated prism, and the convex surfaceshape is set to a substantially cylindrical surface shape under thecondition that the value (r/P) of the radius of curvature r normalizedby the arrangement pitch P of the elongated prisms is equal to 2 to 80.In an aspect of the present invention, the ratio (d/P) of the maximumdistance between the convex-surface-shaped prism face of each of theelongated prisms of the light deflecting device and the virtual prismface of each virtual elongated prism to the arrangement pitch P of theelongated prisms is equal to 0.05 to 5%. In an aspect of the presentinvention, a slant angle of a virtual prism face of each of the virtualelongated prisms at the near side to the primary light source is 45° ormore. In an aspect of the present invention, each of the virtualelongated prisms has an equilateral triangular cross-section. In anaspect of the present invention, the light emission face of the lightguide and/or the back surface of the light guide at the opposite side tothe light emission face is a surface having a directive light emissionfunction. In an aspect of the present invention, an average slant angleof the surface having the directive light emission function is 0.5 to15°. In an aspect of the present invention, the surface having thedirective light emission function is a roughened surface or a surfacehaving many elongated lens arranged thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing a planar light sourcedevice according to the present invention;

FIG. 2 is a diagram showing the shape of elongated prisms on the lightincident surface of a light deflecting device; and

FIG. 3 is a schematic perspective view showing a planar light sourcedevice according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments according to the present invention will bedescribed hereunder with reference to the accompanying drawings.

FIG. 1 is a schematic perspective view showing an embodiment of a planarlight source device according to the present invention.

As shown in FIG. 1, the planar light source device of the presentinvention comprises a light guide having a light incident face 31 on atleast one side end surface thereof, and a light emission face 33 on asurface thereof which is substantially perpendicular to the lightincident face 31, a primary light source 1 that is disposed so as toconfront the light incident face 31 of the light guide 3 and covered bya light source reflector 2, a light deflecting device 4 disposed on thelight emission face of the light guide 3, and a light reflecting element5 disposed so as to confront the back surface 34 of the light guide 3which is at the opposite side to the light emission face 33.

The light guide 3 is disposed in parallel to the XY plane, and it isdesigned in a rectangular plate shape as a whole. The light guide 3 hasfour side end surfaces, and at least one side end surface of a pair ofside end surfaces parallel to the YZ plane is set as the light incidentface 31. The light incident face 31 is disposed so as to confront thelight source 1, and light emitted from the light source 1 is incident onthe light incident face 31 and introduced into the light guide 3. In thepresent invention, a light source may be disposed at the other side endsurface (side end surface 32 or the like) confronting the light incidentface 31.

The two principal surfaces which are substantially perpendicular to thelight incident face 31 of the light guide 3 are located substantially inparallel to the XY plane, and any one of the surfaces (the upper surfacein FIG. 1) is set as the light emission face 33. A directive lightemission function structure comprising a roughened surface or adirective light emission function structure comprising a lens faceachieved by disposing many elongated lenses such as elongated prisms,lenticular lenses, V-shaped grooves or the like substantially inparallel to the light incident face 31 is disposed on at least one ofthe light emission face 33 and the back surface 34, whereby light havingdirectivity in a light emission distribution on the plane (XZ plane)perpendicular to the light incident face 31 and the light emission face33 is emitted from the light emission face 33 while the light incidenton the light incident face 31 is guided through the light guide 3. Theangle at which the direction of the peak in the light emissiondistribution on the XZ plane intersects to the light emission face 31 isrepresented by α. The angle α is preferably set to 10 to 40°, and thehalf-width of the light emission distribution is preferably equal to 10to 40°.

With respect to the roughened surface or the elongated lenses formed onthe surface of the light guide 3, from the viewpoint of keepinguniformity of luminance on the light emission face 33, it is preferablethat the average slant angle θ a based on ISO 4287/1-1984 is set to avalue in the range from 0.5 to 15°. The average slant angle θ a is morepreferably set to a value in the range from 1 to 12°, and furtherpreferably it is set to a value in the range from 1.5 to 11°. It ispreferable that the optimum range is set for the average slant angle θ ain accordance with the ratio (L/t) of the thickness (t) of the lightguide 3 and the length (L) thereof in the direction along which theincident light propagates. That is, when a light guide having L/t ofabout 20 to 200 is used, the average slant angle θ a is preferably setin the range from 0.5 to 7.5°, more preferably it is set in the rangefrom 1 to 5°, and further more preferably it is set in the range from1.5 to 4°. When a light guide 3 having L/t of about 20 or less is used,the average slant angle θ a is preferably set in the range from 7 to12°, and more preferably it is set in the range from 8 to 11°.

The average slant angle θ a of the roughened surface formed on the lightguide 3 can be determined as follows. That is, according toIS04287/1-1984, the shape of the roughened surface is measured by usinga probe type surface roughness tester, and the average slant angle θ ais calculated from the slant function f(x) (x represents the coordinatein the measurement direction) thus achieved by using the followingequations (1) and (2). Here, L represents the measurement length, and Δarepresents the tangent of the average slant angle θ a.Δa=(1/L)∫_(O) ^(L)|(d/dx)f(x)|  (1)θa=tan⁻¹(Δa)  (2)

With respect to the light guide 3, the light emission rate of the lightguide 3 is preferably set in the range of 0.5 to 5%, and it is morepreferably set in the range from 1 to 3%. As the light emission rate isless than 0.5%, the amount of light emitted from the light guide 3 isreduced, and thus there is a tendency that sufficient luminance is notachieved. On the other hand, if the light emission rate is larger than5%, a large amount of light is emitted in the neighborhood of the lightsource 1, and attenuation of light in the X-direction on the lightemission face 33 is remarkable, so that there is a tendency that theuniformity of the luminance on the light emission face 33 is reduced.Therefore, light having such an emission characteristic of highdirectivity that the angle α of the peak light in the light emissiondistribution of light emitted from the light emission face is within therange from 10 to 40° and the half-width of the light emissiondistribution on the XZ plane perpendicular to the light incident faceand the light emission face is within the range from 10 to 40° can beemitted from the light guide 3 by setting the light emission rate of thelight guide 3 in the range from 0.5 to 5% as described above, and theemission direction can be efficiently deflected by the light deflectingdevice 4, thereby providing a planar light source device having highluminance.

In the present invention, the light emission rate of the light guide 3is defined as follows. The relationship between the light intensity(I_(o)) of the emitted light at the end edge of the light emission face33 at the light incident face 31 side and the light intensity (I) of theemitted light at a distance L from the end edge of the light emissionface 33 at the light incident face 31 side satisfies the followingequation (3) wherein t represents the thickness of the light guide (thedimension in the Z direction).I=I _(o) ·a(1−a)^(L/t)  (3)

Here, the constant α represents the light emission rate, and it meansthe rate (%) of light emission from the light guide 3 per unit length(the length corresponding to the thickness t of the light guide) on thelight emission face 33 in the X direction perpendicular to the lightincident face 31. The logarithm of the light intensity of the emittedlight from the light emission face 23 is plotted on the axis ofordinates and (L/t) is plotted on the axis of abscissas, and the lightemission rate α can be calculated from the gradient of the plot result.

In order to control the directivity of the emitted light from the lightguide 3 on a plane (YZ plane) parallel to the light source 1, it ispreferable that a lens face having many elongated lenses which arearranged on the lens face and extend in the direction (X direction)substantially perpendicular to the light incident face 31 is formed onthe other principal surface to which no directive light emissionfunction structure is provided. In the embodiment shown in FIG. 1, theroughened surface is formed on the light emission face 33, and the lensface having the many elongated lenses arranged thereon so as to extendin the direction (X direction) substantially perpendicular to the lightincident face 31 is formed on the back surface 34. In the presentinvention, contrary to the embodiment of FIG. 1, the lens face may beformed on the light emission face 33 while the roughened surface isformed on the back surface 34. As shown in FIG. 1, when the elongatedlenses are formed on the back surface 34 or the light emission face 33of the light guide 3, elongated prisms, lenticular lenses, V-shapedgrooves each of which extends substantially in the X direction may beused as the elongated lenses, and it is preferable that elongated prismseach having substantially triangular shape in section are used.

In the present invention, when the elongated prisms are formed as theelongated lenses on the light guide 3, the vertical angle thereof ispreferably set in the range from 70 to 150°. If the vertical angle isset in this range, the emitted light from the light guide 3 can besufficiently concentrated and the luminance as the planar light sourcedevice can be sufficiently enhanced. That is, by setting the verticalangle of the elongated prism within this range, this device can emitconcentrated light which has the half-width of the light emissiondistribution in the range from 35 to 65° on the plane containing thepeak light of the light emission distribution and perpendicular to theXZ plane, thereby enhancing the luminance as the planar light sourcedevice. When the elongated prisms are formed on the light emission face33, the vertical angle thereof is preferably set in the range from 80 to100°. When the elongated prisms are formed on the back surface 34 isformed on the back surface 34, the vertical angle thereof is preferablyset in the range from 70 to 80° or in the range from 100 to 150°.

In the present invention, in place of or in combination with theformation of the light emission function structure on the light emissionface 33 or the back surface 34 as described above, light diffusing fineparticles may be mixed and dispersed in the light guide to bring thedirective light emission function to the light guide. Further, the shapeof the light guide 3 is not limited to that of FIG. 1, and variousshapes such as a wedge-shape, a boat-shape, etc. may be used.

The light deflecting device 4 is disposed on the light emission face 33of the light guide 3. The two principal surfaces 41, 42 of the lightdeflecting device 4 are confronted to each other, and located inparallel to the XY plane as a whole. One of the principal surfaces whichfaces the light emission face 33 of the light guide is set as a lightincident surface 41, and the other principal surface is set as a lightemission surface 42. The light emission surface 42 is designed as a flatsurface parallel to the light emission face 33 of the light guide 3. Thelight incident surface 41 is designed as a prism-formed surface havingmany elongated prisms which are arranged in parallel to one another soas to extend in the Y direction. The prism-formed surface may beprovided with a flat portion having a relatively small width between therespective neighboring elongated prisms (for example, a flat portionhaving a width that is substantially equal to the pitch of the elongatedprisms or less). However, from the viewpoint of the using efficiency oflight, it is preferable that the elongated prisms are formedcontinuously without providing any flat portion.

FIG. 2 is a diagram showing the shape of each elongated prism of thelight incident surface 41 of the light deflecting device 4. The shape ofeach elongated prism of the light incident surface 41 is set as follows.

That is, the arrangement pitch of the elongated prisms is set to P, anda virtual elongated prism I having a triangular shape in section isfirst set. The intersecting angle between the two virtual prism facesI-1 and I-2 of the virtual elongated prism I (that is, the verticalangle of the virtual elongated prism) is set to θ. The virtual prismvertical angle θ is set so that the peak emitted light (slant angle α)in the intensity distribution on the XZ plane of light coming from thelight emission face 33 of the light guide 3 is incident on the virtualelongated prism I, internally totally reflected by the virtual prismface I-2 and then propagates in the normal direction for example of thelight emission surface 42. When the peak emitted light of light emittedfrom the light emission surface 42 of the light deflecting device 4 isdirected approximately to the normal direction of the light emissionsurface 42 (for example, within the range of ±10° from the normaldirection), the virtual prism vertical angle θ is preferably set in therange from 50 to 70°, more preferably in the range from 55 to 70°, andfurther more preferably in the range from 60 to 70°. Further, in orderto efficiently deflect the emitted light from the light guide 3 to adesired direction by the light deflecting device 4, the slant angle (anintersecting angle to the light emission surface 42) of one virtualprism face of the virtual elongated prism is preferably set to 45° ormore, more preferably to 47° or more, and further more preferably to 50°or more.

Next, by setting as a standard the shape of the virtual elongated prismI for which the shape is set as described above, the shape of an actualelongated prism is set so that one prism face has a convex-shaped curvedsurface. Specifically, the shape of the actual elongated prism ispreferably set as follows. There is considered such virtual light thatthe peak emitted light (slant angle α) in the light emissiondistribution of light emitted from the light emission face 33 of thelight guide 3 skirts the apex portion of a neighboring virtual elongatedprism at the primary light source 1 side and then incident on thevirtual prism I. Here, the position at which the virtual light passesover the virtual prism face I-1 is represented by K1, and the positionat which the virtual light reaches the virtual prism face I-2 isrepresented by K2.

Typically, it is preferable that the whole surface of a part of theactual prism face which is located to be nearer to the light emissionsurface 42 than the position K2 is shaped like a convex surface. On theother hand, the surface of the other area of the actual prism face whichis located to be nearer to the light incident surface 41 (that is,farther from the light emission surface 42) than the internal totalreflection position K2 of the virtual prism face 1-2 (hereinafterreferred to as “area below K2”) may be shaped like a flat surface orconvex surface. In any case, it is preferable to extend the shape of theactual prism face in the neighborhood of the light emission surface 42side over the position K2. The apex portion of the actual elongatedprism is not required to be coincident with the apex portion of thevirtual elongated prism.

At the position nearer to the light emission surface 42 than theinternal total reflection position K2 of the virtual prism face I-2 inthe virtual elongated prism I, the shape of the actual elongated prismis preferably set to such a convex surface shape that at least a part orthe whole part thereof has a prism-face slant angle larger than theslant angle of the virtual prism face I-2 of the virtual elongated prismI.

This means that at the positions in the Z direction at which thedimension z shown in FIG. 2 (the distance in the Z direction between theapex point of the elongated prism and the internal reflection positionK2 of the virtual prism face I-2) is larger than the value indicated bythe following equation:z={(P·tan α·cot[θ/2])/(tan α+cot[θ/2])}[cot θ/2]+{cot θ/(cot[θ/2]−cotθ)}],the actual prism face has a slant angle larger than that of the virtualprism face I-2 of the virtual elongated prism I indicated by thefollowing equation:ncos[3θ/2] sin(α−[θ/2])(in the equations, n represents the refractive index of the elongatedprism). The above positions in the Z direction above the position K2will be hereinafter referred to as “area above K2”.

By setting the shape of the elongated prism of the light incidentsurface 41 as described above, the distribution angle (half-width) ofthe light emitted from the light deflecting device 4 can be reduced. Thereason is as follows. That is, light reaching the positions nearer tothe light emission surface 42 than the internal total reflectionposition K2 of the virtual prism face I-2 of the virtual elongated prismI (i.e., the area above K2) is a flux of light beams passing through alower position than the apex portion of the neighboring virtualelongated prism at the primary light source side at a slant angle largerthan a. Accordingly, the direction of the distribution peak thereofcorresponds to a direction of slant angle larger than α, and thedirection of the distribution peak of the internally totally reflectedlight thereof corresponds to a direction that is slant from the normaldirection of the light emission surface 42 toward the direction alongthe virtual prism face of the internal total reflection. Such light actsto expand the angular distribution of the emitted light from the lightemission surface 42. Therefore, in order to emit light whileconcentrating the light amount to a specific direction, in at least apart of the area nearer to the light emission surface 42 than theinternal total reflection position K2 of the virtual prism face I-2 ofthe virtual elongated prism I (i.e., the area above K2), the slant angleof the prism face of the actual elongated prism is set to be larger thanthe slant angle of the corresponding virtual prism face, whereby thepropagating direction of light which is actually internally totallyreflected in this area can be corrected to be more moved toward thenormal direction of the light emission surface 42 than the virtualreflected light from the virtual prism face, so that the luminance canbe enhanced and the visual field can be narrowed.

The convex surface as described above may be formed in the whole part ofthe area (area above K2) nearer to the light emission surface 42 thanthe internal total reflection position K2 of the virtual prism face I-2of the virtual elongated prism I while the shape of the virtual prismface I-2 of the virtual elongated prism is left without any modificationin the area which is farther from the light emission surface 42 than theinternal total reflection position K2 (i.e., the area below K2).Alternatively, the whole prism surface containing the area below K2 maybe designed as a convex surface. As such a convex surface shape may beused a convex cylindrical surface shape having a radius of curvature rin which at least base portion is common to the virtual elongated prism.

Here, the value (r/P) of the radius of curvature r normalized by thepitch P is preferably set in the range from 2 to 80, more preferably inthe range from 7 to 30, further more preferably in the range from 7.5 to20, and specifically in the range from 8 to 15. By setting the value ofr/P in this range, the half-width of the light emission distribution oflight emitted from the light emission surface 42 of the light deflectingdevice 4 can be sufficiently narrowed, and the luminance as the lightsource device can be sufficiently increased. For example, when the pitchof the elongated prisms is set to 40 to 60 μm, the radius of curvature ris preferably set in the range from 250 to 3000 μm, more preferably inthe range from 350 to 1000 μm and further more preferably in the rangefrom 3.50 to 700 μm.

Further, the convex surface shape of each elongated prism of the lightdeflecting device 4 is preferably set to such a relatively moderatecurved surface that the ratio (d/P) of the maximum distance d betweenthe virtual prism face of the virtual elongated prism and the actualprism face of the convex surface shape and the arrangement pitch P ofthe elongated prisms is equal to a value in the range from 0.05 to 5%,more preferably in the range from 0.1 to 3%, further more preferably inthe range from 0.2 to 2%, and specifically in the range from 0.7 to1.5%. If the value of d/P is more than 5%, there is such a tendency thatthe light concentration effect by the light deflecting device is lostand divergence of light occurs. Further, there is also such a tendencythat the half-width of the light emission distribution of light emittedfrom the light emission surface 42 of the light deflecting device 4cannot be sufficiently narrowed. Conversely, if the value of d/P is lessthan 0.05%, there is such a tendency that the light concentration effectby the light deflecting device 4 is lowered and there is also such atendency that the half-width of the light emission distribution of lightemitted from the light emission surface 42 of the light deflectingdevice 4 cannot be sufficiently narrowed.

In the present invention, the convex surface shape of each elongatedprism of the light deflecting device 4 is not limited to the arcuateshape in section having the radius of curvature r as described above,and any non-spherical convex surface shape may be used insofar as thevalue of d/P thereof is within the above range.

In the present invention, the prism face having the convex surface shapeas described above is preferably formed in each elongated prism at leaston a surface which is far away from the primary light source 1. Thisarrangement can reduce the distribution angle of light emitted from thelight deflecting device 4 when a primary light source is also disposedon the end surface 32 of the light guide 3. When the rate at which lightpropagating through the light guide 3 is reflected by and returns fromthe end surface 32 at the opposite side to the light incident face 31 isrelatively high, the prism face located at the side near to the primarylight source 1 is preferably designed to have a convex surface shape.Particularly, the prism face located at the side near to the primarylight source I is preferably shaped to be symmetrical with the actualprism face corresponding to the virtual prism face I-2 with respect tothe normal direction of the light emission surface 42. On the otherhand, when the rate at which the light propagating through the lightguide 3 is reflected by and returns from the end surface 32 at theopposite side to the light incident face 31 is relatively low, the prismface at the side near to the primary light source 1 may be designed tohave a flat surface. Further, when it is required to sharpen the apexportions of the elongated prisms (form the edges of the apex portionsclearly) in order to suppress occurrence of a sticking phenomenon whenthe light deflecting device 4 is mounted on the light guide 3, it ispreferable that the prism face at the side near to the primary lightsource 1 is designed to have a flat surface because the shape of ashape-transferring face of a molding member for forming the elongatedprisms can be more accurately formed as compared with a case where boththe prism faces are designed to have a convex surface, and thus the apexportions of the elongated prisms can be more easily sharpened.

In the light deflecting device of the present invention, in order toaccurately form a desired prism shape to achieve stable opticalperformance, and to suppress abrasion and deformation of the apexportions of elongated prisms when a fabricating work is carried out orthe device is used as a planar light source device, a flat portion or acurved surface portion may be formed at the apex portion of eachelongated prism. In this case, the width of the flat portion or thecurved surface portion formed at the apex portion of each elongatedprism is preferably set to 3 μm or less from the viewpoint ofsuppressing reduction of luminance as a planar light source device oroccurrence of a non-uniform pattern of luminance due to the stickingphenomenon, and it is more preferably set to 2 μm, and further morepreferably it is set to 1 μm or less.

In the present invention, in order to adjust the visual field angle andenhance the quality as the planar light source device, a light diffusionlayer may be formed at the light emission surface side of the lightdeflecting device or light diffusing agent may be contained in theelongated prisms. The light diffusion layer may be formed by disposing alight diffusion sheet on the light emission surface of the lightdeflecting device or integrally forming a light diffusion layer with thelight deflecting device at the light emission surface side. In thiscase, in order to suppress degradation of the luminance enhancing effectdue to the narrowing of the visual field by the light deflecting deviceat maximum, it is preferable that a light diffusion layer havinganisotropic diffusing property is formed to diffuse light in a desireddirection. Transparent fine particles having a refractive indexdifferent from that of the elongated prisms may be used as the lightdiffusing agent to be dispersed in the elongated prisms. In this case,in order to suppress degradation of the luminance enhancing effect dueto the narrowing of the visual field by the light deflecting device atmaximum, the content, the particle size, the refractive index, etc. ofthe light diffusing agent are suitably set.

As described above, by mounting the light deflecting device 4 describedabove on the light emission face 33 of the light guide 3 so that theprism-formed surface thereof corresponds to the light incident side, thelight emission distribution on the XZ plane of the directive emittedlight emitted from the light emission face 33 of the light guide 3 canbe more narrowed, and the luminance as the light source device can beenhanced, and the visual field can be narrowed. The half-width of thelight emission distribution on the XZ plane of the light emitted fromthe light deflecting device 4 as described above is preferably set inthe range from 5 to 25°, more preferably in the range from 10 to 20′,and further more preferably in the range from 12 to 18°. By setting thehalf-width of the light emission distribution to 5° or more, unclearnessof images, etc. due to excessively narrowing of the visual field can beprevented, and by setting the half-width of the light emissiondistribution to 25° or less, the luminance can be enhanced and thevisual field can be narrowed.

The narrowing of the visual field by the light deflecting device 4 ofthe present invention is affected by the spreading degree (half-width)of the light emission distribution (within the XZ plane) of lightemitted from the light emission face 33 of the light guide 3, and thusthe rate of the half-width A of the light emission distribution of lightemitted from the light emission surface 42 of the light deflectingdevice 4 to the half-width B of the light emission distribution of lightemitted from the light emission face 33 of the light guide 3 is variedin accordance with the half-width B of the light emission distributionof light from the light guide 3. For example, when the half-width B ofthe light emission distribution of light from the light guide 3 is lessthan 26°, the half-width A is preferably set in the range from 30 to 95%with respect to the half-width B, more preferably in the range from 30to 80%, and further more preferably in the range from 30 to 70%. Whenthe half-width B of the light emission distribution of light from thelight guide 3 is not less than 26°, the half-width A is preferably setin the range from 30 to 80% with respect to the half-width B, morepreferably in the range from 30 to 70%, and further more preferably inthe range from 30 to 60%. Particularly when the half-width B of thelight emission distribution of light from the light guide 3 is in therange from 26 to 36°, the half-width A is preferably set in the rangefrom 30 to 80% with respect to the half-width B, more preferably in therange from 30 to 70%, and further more preferably in the range from 30to 60%. Further, when the half-width B of the light emissiondistribution of light from the light guide 3 exceeds 36°, the half-widthA is preferably set in the range from 30 to 70% with respect to thehalf-width B, more preferably in the range from 30 to 60%, and furthermore preferably in the range from 30 to 50%.

As described above, according to the present invention, as thehalf-width of the light emission distribution of light from the lightguide 3 increases, the effect of the narrowing of the visual field isenhanced. Therefore, from the viewpoint of the efficiency of thenarrowing of the visual field, the light deflecting device is preferablyused in combination with a light guide having the half-width B of thelight emission distribution of 26° or more, and it is more preferablyused in combination with a light guide having the half-width B of thelight emission distribution more than 36°. When the half-width of thelight emission distribution of light from the light guide 3 is small,the effect of the narrowing of the visual field is lowered. However, asthe half-width of the light emission distribution of light from thelight guide 3 is reduced, the luminance is enhanced. Therefore, from theviewpoint of enhancing the luminance, the light deflecting device ispreferably used in combination with a light guide having the half-widthB of the light emission distribution which is less than 26°.

The primary light source 1 is a linear light source extending in the Ydirection, and a fluorescent lamp or a cold cathode tube may be used asthe primary light source 1. In the present invention, the primary lightsource 1 is not limited to a linear light source, and it may be a pointlight source such as an LED light source, a halogen lamp, a metalhalogen lamp or the like. Particularly when it is used for adisplay-device having a relatively small screen size such as a cellularphone, a portable information terminal or personal digital assistants,or the like, a small point light source such as LED or the like ispreferably used. The primary light source 1 may be mounted not only atone side end surface of the light guide 3, but also at the otherconfronting side end surface of the light guide 3 as occasion demands.

The light source reflector 2 is used to guide the light from the primarylight source 1 to the light guide 3 with reduced loss. Plastic filmhaving a metal deposited reflecting layer formed thereon is used as thematerial of the light source reflector 2. As shown in FIG. 1, the lightsource reflector 2 is folded so as to extend from the outer surface ofthe end edge portion of the light reflecting element 5 through the outersurface of the primary light source 1 to the end edge portion of thelight emission surface of the light deflecting device 4. The lightsource reflector 2 may be folded so as to avoid the light deflectingdevice 4 and extend from the outer surface of the end edge portion ofthe light reflecting element 5 through the outer surface of the primarylight source 1 to the end edge portion of the light emission face of thelight guide 3.

The same reflecting member as the light source reflector 2 may beprovided to a side end surface other than the side end surface 31 of thelight guide 3. A plastic sheet having a metal deposited reflecting layerformed thereon may be used as the light reflecting element 5. In thepresent invention, a light reflecting layer may be formed as the lightreflecting element 5 by metal deposition or the like on the back surface34 of the light guide 3 in place of the reflecting sheet.

The light guide 3 and the light deflecting device 4 of the presentinvention may be formed of synthetic resin having high lighttransmittance. As the synthetic resin may be used methacrylic resin,acrylic resin, polycarbonate resin, polyester resin, vinyl chlorideresin, or the like. The methacrylic resin is optimum because it has highlight transmittance and is excellent in heat-resistance, dynamiccharacteristics, molding processability. It is preferable thatmethacrylic resin contains methyl methacrylate as main component byweight % of 80 or more.

The surface structures of the roughened surface or the elongated prism,etc. of the light guide 3 and the light deflecting device 4 are achievedby thermally pressing a transparent synthetic resin with a moldingmember having a desired surface structure. The shaping may be carriedout simultaneously with the molding by using screen print, extrusionmolding, injection molding or the like. Alternatively, the surfacestructure may be achieved by using thermosetting or photocurable resinor the like. Further, a roughened surface structure or an elongated lensarrangement structure formed of active energy curable resin may beformed on the surface of a transparent base member such as a transparentfilm or sheet of polyester resin, acrylic resin, polycarbonate resin,vinyl chloride resin, polymethacrylic imide resin or the like, or such asheet may be integrally joined onto another transparent base member byadhesion, fusion or the like. Multi-functional (met)acrylic compounds,vinyl compounds, (met)acrylic esters, allyl compounds, metallic salts of(met)acrylic acid may be used as the active energy curable resin.

A liquid crystal device is mounted on the light emission surface (thelight emission surface 42 of the light deflecting device 4) of theplanar light source device comprising the primary light source 1, thelight source reflector 2, the light guide 3, the light deflecting device4 and the light reflecting element 5 as described above to fabricate aliquid crystal display device. The liquid crystal display device isviewed through the liquid crystal device from the upper side of FIG. 1by a viewer. In the present invention, light having a sufficientlynarrow distribution can be irradiated from the planar light sourcedevice to the liquid crystal device, so that gradation inversion, etc.can be prevented in the liquid crystal device and an image havingexcellent uniformity in luminance and hue can be displayed. In addition,light can be concentratively irradiated in a desired direction, and theusing efficiency of the amount of emitted light from the primary lightsource for illumination in this direction can be enhanced.

FIG. 3 is a schematic perspective view showing another embodiment of theplanar light source device of the present invention. This embodiment isdifferent from the embodiment shown in FIGS. 1 to 2 only in that theback surface 34 of the light guide 3 is designed as a flat surface, thelight guide 3 is designed in a wedge form so that the thickness thereofis gradually reduced from the light incident end face 31 to the endsurface 32 at the opposite side, and a light shielding member 6 isdisposed to prevent bright line and dark line in the neighborhood of theprimary light source 1.

The above embodiments relate to the planar light source device, however,the present invention may be applied to a light source device that isshaped like a slender rod extending in the X direction and has adimension in the Y direction which is five times or less of thethickness of the light guide 3. In this case, a substantially point typelight source such as LED or the like may be used as the primary lightsource 1.

Next, the present invention will be described in more detail by usingExamples. The measurement of the respective physical properties in theExamples was carried out under the following conditions.

Measurement of Luminance in Normal Direction and Half-width of LuminousIntensity of Planar Light Source Device

A cold cathode tube was used as the light source, it was turned on athigh frequency by applying DC 12V to an inverter (HIU-742A produced byHarison Electric Co., Ltd.) connected thereto. With respect to theluminance, the surface of the planar light source device or the lightguide was divided into 3×5 squares each having 20 mm×20 mm size, and theaverage value of luminance values in the normal direction which weremeasured for the fifteen squares was calculated. With respect to thehalf-width of luminous intensity, black paper having a pin hole of 4 mmφwas fixed on the surface of the planar light source device or the lightguide so that the pin hole was located at the center of the planar lightsource device or the light guide, the distance between a luminance meterand the planar light source device or the light guide was adjusted sothat the measurement circle of the luminance meter was equal to 8 to 9mm, and the rotating axis of a goniometer was rotated around the pinhole in both directions perpendicular and parallel to the longitudinaldirection of the cold cathode tube. The distribution of luminousintensity of emitted light was measured while the rotating axis wasrotated every 0.5° in the range from +80° to −80° in each of thedirections, and the luminance in the normal direction and the half-width(the spreading angle of the distribution at a half value of the peakvalue) of the luminous intensity distribution were determined.

Measurement of Average Slant Angle (θa)

According to IS04287/1-1987, the surface roughness of the roughenedsurface was measured at a driving speed of 0.03 mm/second by a probetype surface roughness tester (SURFCOM 570A produced by Tokyo Seiki Co.,Ltd.) using 010-2528 (1 μmR, 55° circular cone, diamond) as a probe. Onthe basis of a chart achieved through this measurement, the average linethereof was subtracted from the chart result to correct the inclination,and the average slant angle was calculated from the above equations (1)and (2).

EXAMPLE 1

A plate-shaped light guide having a mat-finished surface (average slantangle of 3.0 degrees) on one surface thereof was formed by injectionmolding of acrylic resin (ACRYPET VH5#000 produced by Mitsubishi RayonCo., Ltd.). The light guide was a wedged plate of 195 mm×253 mm in sizeand 3 mm-1 mm in thickness. A prism layer comprising elongated prismsmade of acrylic ultraviolet curable resin having a vertical angle of140° and a pitch of 50 μm was formed on the mirror surface side of thelight guide so that the elongated prisms were continuously arranged inparallel to one another and in parallel to one side (short side: 195 mmin length) of the light guide. Further, a cold cathode tube was disposedalong the long side (253 mm in length) of the light guide so as toconfront one side end surface (at a side of light guide's thickness of 3mm) corresponding to the long side of the light guide while it wascovered by a light source reflector (silver reflecting film produced byReiko Co., Ltd.). Further, a light diffusing reflecting film (E60produced by Toray Industries, Inc.) was attached to the other side endsurfaces, and a reflecting sheet was disposed on the elongated prismarrangement (back surface). The structure thus achieved was installed ina frame. With the light guide thus constructed, the light emission ratewas 1.5%, the maximum peak in the luminous intensity distribution ofemitted light was equal to 70° to the normal direction of the lightemission face, and the half-width (half-width B) was equal to 24.5°.

Further, a prism formed surface having many elongated prisms which werecontinuously arranged at a pitch of 50 μm in parallel to one another sothat the whole of each of both the prism faces thereof had a convexsurface shape having a radius of curvature (r) shown in Table 1 wasformed on one surface of polyester film of 50 μm in thickness by usingacrylic ultraviolet curable resin having a refractive index of 1.5064,thereby forming a prism sheet. At this time, as virtual elongated prismswere set elongated prisms which were arranged at a pitch of 50 μm andeach of which had an equilateral triangle in cross section having avertical angle of 65.4° so that light was emitted in the normaldirection to the light emission surface thereof.

The prism sheet thus achieved was disposed so that the prism formedsurface faced the light emission face of the light guide and theridgelines of the elongated prisms were in parallel to the lightincident face of the light guide. The relative intensity in the peakluminance and the half-width (half-width A) in the light emissiondistribution on the plane perpendicular to the cold cathode tube in theplanar light source device thus manufactured were determined, and theresult is shown in the Table 1.

COMPARATIVE EXAMPLE 1

In the same manner as EXAMPLE 1 except that the prism faces constitutingeach elongated prism of the prism sheet were flat, a prism sheet havingelongated prisms that were arranged at a pitch of 50 μm and respectivelyhad an equilateral triangle in cross section having a vertical angle of65.4 degrees was formed on one surface thereof. The prism sheet thusachieved was mounted so that the prism formed surface thereof faced thelight emission face of the light guide achieved in EXAMPLE 1 and theprism ridgelines thereof were in parallel to the light incident face ofthe light guide. The relative intensity in the peak luminance and thehalf-width (half-width A) in the light emission distribution on theplane perpendicular to the cold cathode tube in the planar light sourcedevice thus manufactured were determined, and the result is shown in theTable 1. TABLE 1 Radius of Maximum Relative Half-width A/ Curvature rDistance d d/P Peak Half-width A Half-width B (μm) r/P (μm) (%)Intensity (deg.) (%) Ex. 1 A 250 5.00 1.07 2.14 1.10 20.37 83 B 375 7.500.71 1.43 1.33 15.06 61 C 400 8.00 0.67 1.34 1.44 14.75 60 D 425 8.500.63 1.26 1.46 14.78 60 E 450 9.00 0.59 1.19 1.44 14.82 60 F 475 9.500.56 1.13 1.44 14.86 61 G 500 10.00 0.54 1.07 1.46 14.98 61 H 525 10.500.51 1.02 1.45 15.09 62 I 550 11.00 0.49 0.97 1.42 15.26 62 J 575 11.500.47 0.93 1.40 15.44 63 K 600 12.00 0.45 0.89 1.40 15.67 64 L 625 12.500.43 0.86 1.38 15.86 65 M 750 15.00 0.36 0.71 1.33 16.51 67 N 1250 25.000.22 0.43 1.19 18.72 76 O 2500 50.00 0.11 0.22 1.07 20.61 84 P 300060.00 0.09 0.18 1.05 21.80 89 Q 4000 80.00 0.07 0.14 1.04 23.02 94 Com.Ex. 1 — — 0.00 0.00 1.00 27.00 110

EXAMPLE 2

A plate-shaped light guide having a mat-finished surface (average slantangle of 8.0 degrees) on one surface thereof was formed by injectionmolding of acrylic resin (ACRYPET VH5#000 produced by Mitsubishi RayonCo., Ltd.). The light guide was a wedged plate of 195 mm×253 mm in sizeand 3 mm-1 mm in thickness. A prism layer comprising elongated prismsmade of acrylic ultraviolet curable resin having a vertical angle of140° and a pitch of 50 μm was formed on the mirror surface side of thelight guide so that the elongated prisms were continuously arranged inparallel to one another and in parallel to one side (short side: 195 mmin length) of the light guide. Further, a cold cathode tube was disposedalong the long side (253 mm in length) of the light guide so as toconfront one side end surface (at a side of light guide's thickness of 3mm) corresponding to the long side of the light guide while it wascovered by a light source reflector (silver reflecting film produced byReiko Co., Ltd.). Further, a light diffusing reflecting film (E60produced by Toray Industries, Inc.) was attached to the other side endsurfaces, and a reflecting sheet was disposed on the elongated prismarrangement (back surface). The structure thus achieved was installed ina frame. With the light guide thus constructed, the light emission ratewas 4.5%, the maximum peak in the luminous intensity distribution ofemitted light was equal to 61° to the normal direction of the lightemission face, and the half-width (half-width B) was equal to 39′.

Further, a prism formed surface having many elongated prisms which werecontinuously arranged at a pitch of 50 μm in parallel to one another sothat the whole of each of both the prism faces thereof had a convexsurface shape having a radius of curvature (r) shown in Table 2 wasformed on one surface of polyester film of 50 μm in thickness by usingacrylic ultraviolet curable resin having a refractive index of 1.5064,thereby forming a prism sheet. At this time, as virtual elongated prismswere set elongated prisms which were arranged at a pitch of 50 μm andeach of which had an equilateral triangle in cross section having avertical angle of 65.4° so that light was emitted in the normaldirection to the light emission surface thereof.

The prism sheet thus achieved was disposed so that the prism formedsurface faced the light emission face of the light guide and theridgelines of the elongated prisms were in parallel to the lightincident face of the light guide. The relative intensity in the peakluminance and the half-width (half-width A) in the light emissiondistribution on the plane perpendicular to the cold cathode tube in theplanar light source device thus manufactured were determined, and theresult is shown in the Table 2.

COMPARATIVE EXAMPLE 2

In the same manner as EXAMPLE 1 except that the prism faces constitutingeach elongated prism of the prism sheet were flat, a prism sheet havingelongated prisms that were arranged at a pitch of 50 μm and respectivelyhad an equilateral triangle in cross section having a vertical angle of65.4 degrees was formed on one surface thereof. The prism sheet thusachieved was mounted so that the prism formed surface thereof faced thelight emission face of the light guide achieved in EXAMPLE 2 and theprism ridgelines thereof were in parallel to the light incident face ofthe light guide. The relative intensity in the peak luminance and thehalf-width (half-width A) in the light emission distribution on theplane perpendicular to the cold cathode tube in the planar light sourcedevice thus manufactured were determined, and the result is shown in theTable 2. TABLE 2 Radius of Maximum Relative Half-width A/ Curvature rDistance d d/P Peak Half-width A Half-width B (μm) r/P (μm) (%)Intensity (deg.) (%) Ex. 2 A 250 5.00 1.07 2.14 1.07 24.31 62 B 375 7.500.71 1.43 1.24 18.29 47 C 400 8.00 0.67 1.34 1.32 17.82 46 D 425 8.500.63 1.26 1.33 17.53 45 E 450 9.00 0.59 1.19 1.35 17.49 45 F 475 9.500.56 1.13 1.36 17.33 44 G 500 10.00 0.54 1.07 1.37 17.37 45 H 525 10.500.51 1.02 1.38 17.39 45 I 550 11.00 0.49 0.97 1.37 17.49 45 J 575 11.500.47 0.93 1.38 17.80 46 K 600 12.00 0.45 0.89 1.38 18.01 46 L 625 12.500.43 0.86 1.36 18.21 47 M 750 15.00 0.36 0.71 1.27 19.09 49 N 1250 25.000.22 0.43 1.12 21.61 55 O 2500 50.00 0.11 0.22 1.06 23.50 60 P 300060.00 0.09 0.18 1.04 27.30 70 Q 4000 80.00 0.07 0.14 1.02 29.01 74 Com.Ex. 2 — — 0.00 0.00 1.00 31.20 80

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, as at least oneprism face of each elongated prism formed on the light incident surfaceof the light deflecting device is designed in a convex surface shape onthe basis of the shape of the virtual elongated prism that is set as astandard in accordance with the slant angle of the peak emitted lightfrom the light guide, the efficiency at which the light emitted from theprimary light source is concentratively irradiated in a desired viewingdirection (i.e, the using efficiency of the light amount of the primarylight source) is high, and also the light emission surface of the lightdeflecting device is flat, whereby the light deflecting device can besimplified in structure and easily molded. Particularly, according tothe present invention, the prism face on the light incident surface ofthe light deflecting device is designed in such a convex surface shapethat the slant angle of the prism face is larger than the slant angle ofthe virtual prism face in an area nearer to the light emission surfacethan the position of the virtual elongated prism which is set inaccordance with the slant angle of the peak emitted light from the lightguide, whereby the efficiency at which the light emitted from theprimary light source is concentratively emitted in a desired viewingdirection (i.e., the using efficiency of the light amount of the primarylight source) can be enhanced and also there can be provided a lightsource device in which the light emission surface of the lightdeflecting device is designed to be flat and thus the light deflectingdevice can be simplified in structure and easily molded.

1-52. (canceled)
 53. A light deflecting device comprising: a lightincident surface; and a light emission surface at an opposite side tothe light incident surface, wherein plural elongated prisms arranged inparallel to one another are formed on the light incident surface, eachof the elongated prisms has two prism faces, and at least a prism faceof each of the elongated prisms is designed to have a convex surfaceshape set to such a condition that a ratio (r/P) of a radius ofcurvature r of the convex-surface-shaped prism face to an arrangementpitch P of said elongated prisms is equal to 7 to
 30. 54. The lightdeflecting device of claim 53, wherein a ratio (d/P) of a maximumdistance between the convex-surface-shaped prism face of each of saidelongated prisms and a plane including an apex-side edge and a base-sideedge of the convex-surface-shaped prism face to the arrangement pitch Pof said elongated prisms is equal to 0.05 to 5%.
 55. The lightdeflecting device of claim 53, wherein a slant angle of a planeincluding an apex-side edge and a base-side edge of theconvex-surface-shaped prism face of each of said elongated prisms is 45°or more.
 56. The light deflecting device of claim 53, wherein a verticalangle of each of said elongated prisms is set to 50 to 70°.
 57. A lightdeflecting device comprising: a light incident surface; and a lightemission surface at an opposite side to the light incident surface,wherein plural elongated prisms arranged in parallel to one another areformed on the light incident surface, each of the elongated prisms hastwo prism faces, and at least a prism face of each of the elongatedprisms is designed to have a convex surface shape, and wherein a ratio(d/P) of a maximum distance between the convex-surface-shaped prism faceof each of said elongated prisms and a plane including an apex-side edgeand a base-side edge of the convex-surface-shaped prism face to anarrangement pitch P of said elongated prisms is equal to 0.2 to 2%. 58.The light deflecting device of claim 57, wherein the convex surfaceshape of each of said elongated prisms is set to a substantiallycylindrical surface shape under a condition that a ratio (r/P) of aradius of curvature r of the convex-surface-shaped prism face to thearrangement pitch P of said elongated prisms is equal to 2 to
 30. 59.The light deflecting device of claim 57, wherein a slant angle of aplane including an apex-side edge and a base-side edge of theconvex-surface-shaped prism face is 45 or more.
 60. The light deflectingdevice of claim 57, wherein a vertical angle of each of said elongatedprisms is set to 50 to 70°.
 61. A light deflecting device comprising: alight incident surface; and a light emission surface at an opposite sideto the light incident surface, wherein plural elongated prisms arrangedin parallel to one another are formed on the light incident surface,each of the elongated prisms has two prism faces, and at least a prismface of each of said elongated prisms is designed in a convex surfaceshape in at least a part of an area of said prism face.
 62. The lightdeflecting device of claim 61, wherein said prism face is designed inthe convex surface shape in at least a part of an area nearer to thelight emission surface than a position at which light skirting the apexportion of a neighboring one of said elongated prisms to pass in adirection of peak light of a distribution of light incident on anotherprism face of each of said elongated prisms is internally totallyreflected by the prism face of each of said elongated prisms at anopposite side to said another prism face.
 63. The light deflectingdevice of claim 61, wherein the convex surface shape of each of saidelongated prisms is set to a substantially cylindrical surface shapeunder a condition that a ratio (r/P) of a radius of curvature r of theconvex-surface-shaped prism face to an arrangement pitch P of saidelongated prisms is equal to 2 to
 30. 64. The light deflecting device ofclaim 61, wherein a ratio (d/P) of a maximum distance between theconvex-surface-shaped prism face of each of said elongated prisms ofsaid light deflecting device and a plane including an apex-side edge anda base side edge of the convex surface-shaped prism face to anarrangement pitch P of said elongated prisms is equal to 0.05 to 5%. 65.The light deflecting device of claim 61, wherein a slant angle of aplane including an apex-side edge and a base-side edge of theconvex-surface-shaped prism face is 45° or more.
 66. The lightdeflecting device of claim 61, wherein a vertical angle of each of saidelongated prisms is set to 50 to 70°.